This invention relates to multi-computer systems, and more particularly to database management systems within interconnected computer networks.
The term "distributed database management system" as applied here means a computer system for a database management system (hereinafter, "DBMS") involving multiple computer sites, each with a local database connected together in a communications network, in which a user at any site can access data stored at any other site.
Each site, in turn, has a DBMS in its own right: It has its own terminals and users, its own local storage and CPU (central processing unit), running its own database and database administration functions (i.e., a local DBMS). It also has its own data communications manager with the additional responsibility for controlling the exchange of messages with other sites in the overall distributed database system. Taken together, a distributed database management system wherein individual database systems may be from different manufacturers, the overall system is often referred to in the literature as a heterogeneous distributed database management system, or HDBMS. An HDBMS must support various database systems with different database models, languages, and services.
An example of such a system is shown in FIG. 1. The example represents a simple distributed banking system with two sites, for example, one in Portland, Oreg. and one in Washington, D.C. Of course, real distributed systems usually involve more than just two sites. But suppose account records for the Washington, D.C. area are stored in a local database at the D.C. site, while account records for the Oregon area are stored in a local database at the Portland site. Suppose further that the two sites are linked together to form a single "global" or distributed database. The system combines efficiency of processing (the data is stored close to the point where it is most frequently used) with increased accessibility (it is possible to access a Washington, D.C. account from Portland, Oreg., and vice versa, via a communications link).